Commit 6e6f27b6

 made it possible to set the PKG_CONFIG variable using
CROSS_COMPILE as a prefix. The problem with that solution is that it makes
pkgConfigExecutable() skip the environment setup for pkg-config as well,
as it expects the pre-set command to be self-contained - which it isn't.
To avoid this problem we need to store the pkg-config define in the
device spec in a separate variable.

Change-Id: Id8ae7fb03d9253be55840e23fe73b30815ee86c3
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>